In recent years, much attention has been directed to a technique for forming a thin film transistor (TFT) by using a semiconductor thin film (with a thickness of approximately several nanometers to several hundred nanometers) which is formed over a substrate having an insulating surface. Thin film transistors are applied to a wide range of electronic devices such as ICs or electro-optical devices, and there is an expectation for immediate development of thin film transistors to be used, especially, as switching elements for image display devices. Various metal oxides are used for a variety of applications. Indium oxide is a well-known material and is used as a transparent electrode material which is necessary for liquid crystal displays and the like.
Some metal oxides exhibit semiconductor characteristics. Examples of such metal oxides having semiconductor characteristics include tungsten oxide, tin oxide, indium oxide, zinc oxide, and the like. Thin film transistors in which a channel formation region is formed of such metal oxides having semiconductor characteristics are known (Patent Documents 1 and 2).